Phosphorylation of the rRNA transcription factor upstream binding factor promotes its association with TATA binding protein

rRNA synthesis by RNA polymerase I requires both the promoter selectivity f actor 1, which is composed of TATA binding protein (TBP) and three TBP-asso ciated factors, and the activator upstream binding factor (UBF). Whereas th ere is strong evidence implicating a role for phosphorylation of UBF in the control of growth-induced increases in rRNA transcription, the mechanism o f this effect is not known. Results of immunoprecipitation studies with TBP antibodies showed increased recovery of phosphorylated UBF from growth-sti mulated smooth muscle cells. Moreover, using an immobilized protein-binding assay, we found that phosphorylation of UBF in vivo in response to stimula tion with different growth factors or in vitro with smooth muscle cell nucl ear extract increased its binding to TBP. Finally, we demonstrated that UBF -TBP binding depended on the C-terminal 'acidic tail' of UBF that was hyper phosphorylated at multiple serine sites after growth factor stimulation. Re sults of these studies suggest that phosphorylation of UBF and subsequent b inding to TBP represent a key regulatory step in control of growth-induced increases in rRNA synthesis.